<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        .red{
            background-color: red;
        }
    </style>
</head>
<body>
<div id="i1" class="c1">123</div>
<script type="text/javascript" src="jquery-2.1.4.js"></script>
<script type="text/javascript">
    $(function(){
        $('.c1').addClass('red')
    });
</script>
</body>
</html>